Modulation analyzer



Sept. 1, 1936. v D. GlMEsEq-,AL v2,053,076

MODULAT I ON ANALYLER Filed April 251932 Ili- INVENToRs /F-n I -DAVIDGRIMES w1 ATroRNEY Patented Sept. 1, 1936 Y UNITED gsifli'rlas PATENT.OFFICE 2,053,076 l Y n l Island, N. Y., assign ors to Radio Corporationf of America, a corporation of Delaware Application April 26,

8 Claims.

Our present invention relates to methods of measuring percentagemodulation, and more particularly to an improved method of measuringpercentage modulation of modulated signal fi energy.

While it is often convenient to measure radio frequency or intermediatefrequency gain with an unmodulated signal, the case of a modulatedsignal is important because of receiver sensitivity and over-allperformance measurements. For this reason, and due largely to theconsiderable amount of interest which is frequently noted in the matterof signal structure, the present invention concerns itself with signalanalysis in general, and with a method of measuring percentagemodulation in the usual cases. As is well known, a standard modulatedsignal comprises a carrier and two side components whose properties canbe determined by considering an ideal modulator. c

`In such a case, a pure carrier potential is* applied'in the controlgrid circuit of a single stage amplifier whose output circuit is tunedto the carrier frequency, and is sufficiently broad to present a tunedimpedance which is substantially a pure resistance of constant valueover a band of frequencies including the two side components. This idealmodulator is conceived to exhibit a strictly linear relation of radiofrequency output to the low frequency control potential applied forexample in the screen or plate circuit. 'Ihe modulating potential issuperimposed on `the steady screen or .plate voltage.

It can, then, be theoretically and physically demonstrated that in thisideal case the gain of the stage is controlled at the modulatingrfre-rquency, modifying the output at any instant with an intended andsystematical variation of relatively low periodicity, although thisratev of variation need not be restricted to a relatively slow timefunction. Mathematical analysis -of the aforementioned ideal casereveals that modulation has not altered the carrier, but has developedtwo side components which are equal. in magnitude, and bear frequencysymmetry with respect tothe carrier.

This method of considering the effect of modulation on the carrier, withconsequent development of two side components of equal magnitude, haslimited utility, but is a convenience in some cases, especially whereVcertain complications arise. Occasionally, a vectorial conception ofmodulation is of great assistance. Thus, each of thecarriers, upper sidecomponent and lower side component may be represented by a vector 1932,seri-a1 No. 607,536

having a constant magnitude, and rotating with a constant angularvelocity.

It is customary,A and proper, to employ the carrier vector as areference line, consider it to be stationary, and regard the sidecomponent vectors tov rotate in opposite directions with 2m as theirangular velocity; that is to say, these vectors may be considered asrotating at the audio. modulating frequency, instead of at theirabsolute radio frequencies. From this conception there can be derivedthe subsequent conclusion that the aforementionedl threevvectors vhave aresultant vector instantaneous magnitude.

Now, we have found that as far as observable results with any knowndevice as concerned, the degree of modulation can be determined bymeasuring the side components and the carrier, indi` vidually. `Such avmethodis employed in the signal analyzer of the presentinvention whereina piezo-electric crystal is utilized as ameans'of suf? ciently highyselectivity to discriminate-,against the carrier and the lower sidecomponent, `Vfor example, whenthe upper'side component is beingmeasured. vl g Briey, it may be stated that the present'signal analyzerembodies a device kfor adjusting a local oscillator frequencyV to beatwith any desired? signal componentv at the-frequency of crystal resonance. Thus, the `output of a conventional first detector contains aparticular component whichis selected by they crystal, amplified, andmeas'- ured by a tube voltmeter. `This process is per--V Total sidecomponent magnitude y Caz-riet magnitude This fraction is readilyevaluated from the data afforded by the signal analyzer, and, thus, themodulation indicating device Von a signal gener.

ator may be calibrated, vdue regard to certain fol-v lowingconsiderationsbeing paid. Thesel are interesting from the standpoint ofsignal structure in general, as well as from the standpoint ofmodulation measurement.

It iswell recognized that many signal genera` tors depart from idealproduction of a modulated carrier wave. Hence, it is essential toconsider the'eifects of these departures on the simple analyzer methodof measuring percentage modulation, as disclosed in the presentapplication. A signal generator may introduce frequency modulation, inconjunction with the legitimate amplitude modulation, and may introduceside component phase distortion due to resonance phenomena embodied inthe design for good reasons. Harmonics of the modulating tone may bepres-l ent, at the modulator. Any, or all, of these eX- traneous effectsare capable of causing the foregoing definition of M to be in error. f,f

Suppose a signal generatorto have side com-f ponent phase distortion asits only essential defect as far as modulation is concerned, andconsider the modulated signal to exist in a selective circuit with thecarrier on resonance. The side components are off resonance, andtherefore suffer aphase vshift relative to themselves in a pureresistance device. However, it'can be readily demonstrated that the'matter of phase'distortion due to usual tuned circuits, which are purelyresistive to the carrier and resistive and reactivel to the'upper andlower side components, is regarded as being of no consequence. vOi?course, side component attenuation, relative to the carrier in theresonator, decreases the' percentage modulation.

Since phase is ignorable, the analyzer reveals the actualV percentagemodulation of delivered signal. At 400 cycles per second, side componentAattenuation due to resonance is probably unim portantv in most cases.In the oase of 5000 cycles per second modulation, as involved inreceiver over-all fidelity testing, side component attenuationV due toresonance may decrease -the original percentage modulation by 25%. Thepresent analyzer determinesthe actual percentage modulation of the radiofrequency output, which is the desired information.

When harmonics of the modulating toneY are impressed, the system beingotherwise ideal for example, each modulating component sets up a pair ofrradio frequency side components independently of anyV other modulating"components present. It can be shown that in the'absence of frequency, orphase, modulation, the usual harmonic content'pres'ent in the modulatingpotential is not a source of material error, except when receiverover-all distortion measurements are to be taken. In this case theanalyzer of the present invention maybe used to advantage as a ofinvestigation. l n

Of course, pure monotone modulation is striven for. Frequency, or phase,modulation effects are commonly referred to inconnection with signalgenerators, particularly when the oscillator is' modulated directly.These phenomena aremost easily avoided by modulating a radiofrequencystage. The presence of frequencymodulation can be readily determined bythe analyzer described in the'presentapplication, by virtue of unequalmagnitudes of corresponding side components.

It remains onlyl to determine whether or not the foregoing definition ofM is valid when they means Yly that of a standard signal as regardsphase.

In the former case, lower than critical modulating frequency, one of theside components is reversed .in polarity due to over-neutralization.

The variable-modulating frequency procedure is, thus, seen to be aconvenient and trustworthy method for arriving at the orientation whichis in some cases necessary to rigorously arm signal structure, andevaluate M by means of the data afforded by the simple analyzerV of thepresent invention. c Y

Hence, it may no-w be stated that it is one of the prime objects of ourpresent invention to provide a signal structure and percentagemodulation analyzer which essentially consists of a sharplydiscriminating resonator disposed between a frequency changer device anda beat frequencyamplier, the resonator being utilized as a means ofsufficiently high selectivity to discriminate against the carrier andone of the side components when the other side component, for example,is being measured.

Another important object of the present invention is to 4provide asignal wave analyzer, readily adapted for the determination ofpercentag-e modulation of a signal wave, the analyzer being constructedand arranged to measure the ratio of total side component magnitude ofthe Wave to the carrier magnitude of the wave, the said ratio comprisinga measure of the percentage modulation of the wave, the analyzerconsisting of a local oscillator electrically coupled with a frequencychanger, an intermediate frequency amplifier, a subsequent tubevoltmeter, a sharply selective resonator being utilized between theoutput of the frequency changer and the input ofthe intermediatefrequency amplifier. Y

Another object of our present inventionv is to provide a signal waveanalyzer which essentially comprises an electrical receiving circuitoperating :on the heterodyne principle, land utilizing apiezo-electrical coupling device between the frequency changer circuitand the beat frequency utilizing circuit, a pair of coupled resonantcircuits being employed subsequent to the piezoelectric cou'pling toeliminate inherent spurious resonance crystal frequencies separated fromthe desired resonant frequency of the piezo-electric coupling device bya relatively slight number of cycles.

Still other objects of the present invention are to improve generally,lthe simplicity, accuracy,

and efficiency of signal wave analyzers, and to especially provide asignal wave percentage mod-v ulation analyzer which is not only durableand reliable in operation, but economically manufactured and assembled.

The novel features which we believe to be characteristic ofY ourinvention are set Yforth in particularit'y in the appended claims, theinvention itself, however, as to both its organization and method ofoperation will best b-e understood by reference to the followingdescription taken in connection with the drawing in which we haveindicated diagrammatically one circuit arrange- `input to be measured.

ment whereby our invention may be carried into effect. Y

. Referring now to the accompanying drawing, which diagrammaticallyshows an analyzer capable of carrying out theaforementioned objects andpurposes, there is showna pair of terminals I, 2 .adapted to be coupledto the radio frequency In other Words, the terminals I, 2 maybeconnected to the output of a signal generator, or they may be evenconnected to an antenna circuit where the signal wave to be analyzed isradiated from the signal rWave source. An electron discharge tube 3,preferably of the screen grid type, has its control electrode, or grid,adjustably connected tothe terminal I, while the cathode is connected toground through a path including a conductor 4, an inductance coil 5, anda condenser 6, the latter having a magnitude of about 0.1 mi., the condenser being shunted by a resistor 1 having a magnitud-e of about 30,000ohms. They resistor I supplies bias for the grid of tube 3.

A grounded variable resistor 8, functioning as a gain control device, isshunted across the input electrodes of tube 3, the resistor having amagnitude of about 3 megohms, the grid of tube 3 being adjustablyconnected to a'poi'nt on resistor 3. In order to check the drift of theanalyzer, a switch 9. is` connected in shunt with the adjustableresistor 8. The anode oftube 3 is connected to the. positive terminal ofa source of potential (not shown) capable of supplying 135 volts to theanode.

The connection between the said source and the anode of tube 3 comprisesa path consisting of the conductor I0, the resistor II and the conductorI2. The screen grid electrode of the tube 3 is connected, through a pathwhich includes the conductor I3, to a point on the aforementioned sourceof potential which maintains the screen grid electrode at a4 potentialof 67.5 volts. It will be noted that the negative terminal of theaforesaid potential source is noted by the designation B- and that it isconnected to ground.

The tube 3, and its associated circuits, cornprises the frequencychanger device of the analyzer, local oscillations being impressed uponthe frequency changer from a local oscillation source which comprises anelectron discharge tube I 4, preferably a 238 pentode tube having anindirectly heated cathode, it being pointed out that al1 the tubes shownin the present analyzer employ indirectly heated cathodes. The, pentodeoscillator circuit described herein isrmore fully disclosed in acopending application Serial No. 592,461, of W. S. Barden, iiledFebruary 12, 1932. 'I'he anode of tube I4 is connected to be operated ata potential of 135.Volts through a path which includes the conductor I5',. the coil I1, the conductor I3 and the conductor I2. n

'Ihe control electrode of tube I4 is grounded through a path whichincludes the feed-back in- Vductance coil I6 and the lead I5", it beingnoted second' switch 22 is connected in series with the' Verniercondenser 20. The function of these switches, main and Verniercondensers will be described at a later point. Theuse'of the abovevnamed'pentode'` oscillator circuit results in substantial elimination.of undesired second harmonics and. zero grid current flow in theoscil-`lator.y

The cathode of the tube I4 includes in series therewith a grid biasingresistor'24, having a magnitude of about 850 ohms, the resistor beingshunted by a fixed condensery 25 of a magnitude of about 0.2 mf. Thescreen electrode of tube I'4 is connected by a lead I2 to the 135 voltsource, and the suppressor grid is connected to the cathode within thetube in the usual manner. Oscillations produced in the tuned circuitI'I, I8, are impressed on the coil 5, through coupling. M3; thusinjecting the local oscillation frequency into input of tube 3. i

The output circuit of the tube 3 is coupled to the input circuit of theintermediate frequency amplifier tube 26 through a sharply selectiveresonant circuit. The sharply selective resonant circuit comprises aresistor I I, the high potential side of the resistor being connected tothe conductor I Il, and the low potential side of the resistor beingconnected to the conductor I2. The control electrode of tube 26, and theindirectly heated cathode of tube 26, are coupled across resistor I I,by means of a piezo-electric crystal 29, preferably of quartz. Thequartz crystal 29 is cut to be resonant to a frequency of kilocycles,and is disposed between four metallic electrodes, a pair 30 of the saidelectrodes being disposed adjacent one end of the crystal, while theremaining pair 30 are disposed at the opposite end of thecrystal.

The coupling arrangement of the crystal 29, and its location withrespect to the four electrodes are well known to those'skilled intheart, and it is w`e11 recognized that such a coupling arrangement issharply selective with respect. to the-frequency transmitted through thecoupling. The pair of electrodes 30 are connected to opposite sides ofthe resistor II through a fixed direct current blocking condenser 28,each of the fixed condensers having a magnitude of 0.00025 mf. Theremaining pair of electrodes 30 are connected to opposite sides of aresistor 3|, the latter being connected in shunt between the controlgrid andthe grounded side of the cathode of tube 26, the resistor 3|having a magnitude of about 3 megohms.

' The screen grid electrode of tube 26 is connected by means of aconductor 32 to the conductor I3, the latter being connected to thescreen grid electrode of tube 3, and is thus arranged to have a positivepotential of 67.5 volts applied thereto.

It will be noted that the anode of tube 26 Vhas a positive potential of135 volts applied thereto through conductors I2 and v33, the primarycoil 34 of the transformer coupling M2, and the con.- duct'or 35.Theindirectly heatedr cathode of tube 26 includes, in the grounded legthereof, the usual' biasing resistor 36 having a magnitude of about 400ohms, the resistor being shunted by a fixed condenser 3'I havinga'magnitude of 0.1 mf. It will be noted that the anode supply potentiallead lI2 and the screen grid potential supply lead I3 are shunted toground by the fixed condensers' 38 having magnitudes of 0.2 inf. Inorder to preventl undesired radio frequency coupling between the Youtput and input circuits of tubes 3 and 26, there isfdisposed agrounded metallic shield 39 between the electrodes 30 and 30.

The output circuit of tube 26 and the input circuit of the electrondischarge tube 33, which isy preferably a' triode detectorV havingv anindirectly heated cathode, are coupled by a sharply resonant circuitcomprising the primary coil 34 of transformer M2, shunted by anadjustable condenser 40, and the secondary coil 4I shunted by an'adjustable condenser 42. A source of negative grid bias voltage C isconnected between the grounded side of the cathode of tube 39 andthegrid ofthe tube so that the tube 39 and associated circuits function asa second detector. The anode circuit of tube 39 includes themicro-ammeter 43 to indicate the carrier, or side, component magnitudes,the anode being connected by a conductor 44 to a point on the source o'fanode potential which will maintain the anode at volts. The meter 43 isshunted by a variable resistor 45 including in series therewith a sourceof current46.

Considering, now, the operation of the analyzer shown in the drawing anddescribed in detail heretofore, Vthe local oscillator frequency isadjusted by means of the main condenser I8A and either of the Verniercondensers 20 or 'I9 to beat with any desiredV signal componentYimpressed across the input electrodes I, 2, at the frequency of crystalresonance. Thus, assume that a carrier is modulated by a monotone, andthat it is desired toy measure the percentage modulation of theresulting modulated carrier. In the case of more than one modulatingtone, additional side bands can be measured and total modulation.determined. The magnitude of the carrier would first be determined byadjusting the large condenser I8 to the setting which will beat with thecarrier frequency tov produce an intermediate frequency of approximately50 kilocycles, the frequency at which the Width of the crystal 29 isresonant. Any operation of the crystal at the thickness resonance Whichwould be likely to occur at some frequency of the oscillator isprecluded by theV resonant shunt path l0 Il to ground. This path isresonant to the thickness frequency of the crystal. At this setting ofthe condenser I8, the reading on the meter 43 is taken, and this readingindicates the magnitude of the carrier.

It should be noted that each of the condensers 4E) and 42 are adjustedto resonate'their coupled resonant circuits to the desired intermediatefrequency. The function of the coupled resonant circuits between tubes26 and 39 is to remove any spurious responses which might be caused bymultiple crystal resonant frequencies near the legitimate crystalresonance. These undesired resonant peaks result from slightirregularities in grinding of the crystal but are far enough off properresonance to be effectively removed by the resonant coupled circuits.

The tube 39 and its associated circuits actually comprise a tubevoltmeter, Vand the function of the Variable resistor 45 and potentialsource 46 is to Ybalance out of the micro-ammeter 43 all permanentdirect current so that the meter Will be free to indicate slight changesin the plate current caused by the incoming carrier or side bands.Assume, now, that the side component magnitudes are to be measured. Oneof the switches 2l or 22 is closed and the associated vernier condenseris adjusted to the desired side component. At the desired side componentfrequency the reading .of meter 43 is taken. This reading gives themagnitude of said side component frequency without the value of thecarrier, as the carrier frequency is off the crystal resonance frequencyand cannot pass through the crystal.

In this case, the local oscillator has been adjusted to that frequencywhich will beat with the side component frequency to produce the 50kilocycle intermediate frequency. It will be found in employingtheanalyzer that one ofthe Vernier condensers can be employed for thelower frequency side bands, While the Vother one can be Yemployedforthe. higher frequency Aside bands.

After the meter reading of the carrier C has been obtained, andthereadingof the lower side component S1 alone and the .upper sidecomponent S2 have alone been secured, the percentage modulation is equalto: l Y

The square root of eachfdirect current reading is used in this formulabecause the tube 39 is Very nearly a true. square law detectorthroughout the intended range of applied signal intensities.

It Will thus be seen that the output of the first detector tube3contains a component which is selected by the crystal, amplified, andmeasured by the tube voltmeter. This process is performed individuallyon the carrier and each side component, thus determining Vtheir relativemagnitudes. The tube voltmeter may be accurately calibrated. If not,such a'device, when square law, as is usual, gives the squarerof theproper reading. Hence, the actual readings are obtained by a square rootprocess.

While we have indicated and described one arrangement for carrying ourinvention into effect, it will be apparent to one skilled in the artthat our invention is by no means limited to the particular organizationshown and described, but that many modifications may be made withoutdeparting from the scope of our invention as set forth in theappendedclaims.

What We claim is: f

1. A method of determining percentage modulation of amodulated carrierwave which includes beating the modulated Wave with local oscillationsto produce a predetermined beatfrequency, transmitting the beatfrequency energy through a sharply selective path, measuring theintensity of vthe beat frequency energy, and adjusting the frequency oftheA local oscillations toselectively produce said beat frequency energyfor the carrier frequency and theside band frequency in the absenceofthe carrier frequency, consecutively. K

2. Amethod of 'analyzing a modulated carrier wave including heterodyningthe Vcarrierof theV modulated 'carrier wavewith local oscillations of afrequency sufficient to produce a predetermined intermediate frequency,amplifying the latter, detecting the amplified intermediate frequencyenergy, indicating the intensity of the Vdetected energy, andlaterproducing intermediate frequency energy by heterodyning, said localoscillations with either of the side componentsonly of said modulatedWave.

3. A signal `analyzer comprising a'screen grid tube having an inputcircuit and an output circuit, vmeans in `the input circuit of the tubeadapted to be connected to a signal wave so as to be analyzed, a localoscillator comprising a tube having its output circuit coupled to theinput circuit of said screen grid tube, an intermediate frequencyamplifier Y comprising a `screen grid tube providedrwith an inputcircuit and an output circuit, a coupling network connected between theoutputcircut of said first. screen grid tube'and saidsecond screengriditube and comprising a piezo-electric couplingdevice resonant to thedesired intermediate frequency, a tube voltmeter circuit comprising anelectron discharge tube provided With an input circuit coupled to theoutput circuit of said intermediate frequency amplifier tube, the outputcircuit of said electron discharge tube including a visual indicator,

4. A signal analyzer comprising a screen grid tube having anY inputcircuit and an output circuit, means in the input circuit of the tubeadapted to be connected toa signal wave so as to be analyzed, a localoscillator comprising a tube having its output circuit coupled to theinput circuit of said screen grid tube, an intermediate frequencyamplier co-mprising a screen grid tube provided with an input circuitand an output circuit, a -coupling network connected between the outputcircuit of said rst screen grid tube and said second screen grid tubeand comprising a piezo-electric coupling device resonant to the desiredintermediate frequency, a tube voltmeter circuit comprising an electrondischarge tube provided with an input circuit coupled to the outputcircuit of said intermediate frequency amplifier tube, the outputcircuit of said electron discharge tube including a visual indicator,and a tuned circuit in the output circuit of said intermediate frequencyamplifier tube, said last mentioned tuned circuit and the tuned inputcircuit of said electron discharge tube being coupled and resonant tosaid desired intermediate frequency.

5. A signal analyzer comprising a screen grid tube having an inputcircuit and an output circuit, means in the input circuit of the tubeadapted to be connected to a signal Wave so as to be analyzed, a localoscillator comprising a tube having its output circuit coupled to theinput circuit of said screen grid tube, an intermediate frequencyamplifier comprising a screen grid tube provided with an input circuitand an output circuit, a coupling network connected between the outputcircuit of said first screen grid tube and said second screen grid tubeand comprising a piezo-electric coupling device resonant to the desiredintermediate frequency, a tube voltmeter circuit comprising an electrondischarge tube provided with an input circuit coupled to the outputcircuit of said intermediate frequency amplier tube, the output circuitof said electron discharge tube including a visual indicator and a tunedcircuit, resonant to an undesired crystal thick-L ness frequency,connected in the output circuit of said first screen grid tube andarranged in shunt with said piezo-electric coupling device.

6. A signal analyzer comprising a screen grid tube having an inputcircuit and an output circuit, means in the input circuit o-f the tubeadapted to be connected to a signal wave so as to be analyzed, a localoscillator comprising a tube having its output circuit coupled to theinput circuit of said screen grid tube, an intermediate frequencyamplifier comprising a screen grid tube provided with an input circuitand an output circuit, a coupling network connected between the outputcircuit of said first screen grid tube and said second screen grid tubeand comprising a piezo-electric coupling device resonant to the desiredintermediate frequency, a tube voltmeter circuit comprising an electrondischarge tube provided with an input circuit coupled to the outputcircuit ofsaid intermediate frequency amplifier tube, the output circuitof said electron discharge tube including a visual indicator, and saidlocal oscillator tube including in its input circuit a variable tuningcondenser and one or more Vernier condensers.

7. In combination in a percentage modulation analyzer, a source of localoscillations comprise ing an electron discharge tube having an inputcircuit including a device for varying the frequency of oscillationsproduced, a frequency changer circuit including an electron dischargetube having its input circuit coupled to said oscillator output circut,a beat frequency amplifier, a piezo-electric element arranged to couplethe output circuit of said frequency changer tube an-d the input of saidamplifier, said piezo-electric device being sharply resonant to adesired beat frequency, a circuit resonant to a frequency different fromsaid beat frequency coupled to said piezoelectric device and arranged toprevent it from resonating at a frequency other than said beatfrequency, a detector circuit including an ammeter having a graduatedscale in its output, and a pair of resonant circuits, tuned to said beatkfrequency, arranged to couple the output of said amplifier to the inputof said detector circuit, and means connected in the output circuit ofsaid detector for adjusting the operation of said ammeter.

8. A device for measuringthe percentage modulation of a modulatedcarrier wave by comparing the readings of a graduated ammetercomprising, the combination of a vacuum tube frequency changer having aninput and an output circuit, a second detector having an input and anoutput circuit, means for impressing a carrier frequency modulated by asingle audio frequency on the input circuit -of sai-d frequency changer,a source of local oscillations coupled to the input circuit of saidfrequency changer, a circuit network coupling the output circuit of saidfrequency changer to the input circuit of said second detector andincluding a piezo-electric crystal resonant at a relatively lowfrequency and whose selectivity is. such that it will transmit the beatfrequency formed by the local oscillator and carrier frequencies butwill block the beat frequency formed by the local oscillator and the sumfrequency of the carrier and modulation frequencies,

and a graduated ammeter connected in the output circuit of the seconddetector.

DAVID GRIMES. WILLIAM S. BARDEN.

